1. Field of the Invention
This invention relates to an improved process for grafting vinyl chloride onto ethylene-vinyl acetate or ethylene-alkyl acrylate copolymers performed in the gel phase in the presence of radical formers which are soluble in the monomer. More especially this invention relates to the preparation of graft copolymers wherein polyvinyl chloride is grafted onto powdered ethylene-vinyl acetate or ethylene-alkyl acrylate copolymers by a process wherein the basic copolymer is in the form of a powder and, a combination of different initiators are employed, which initiators have different decomposition temperatures from one another. This invention also relates to the resultant graft copolymer and its use as a impact improving agent for polyvinyl chloride homo-or copolymers.
2. Discussion of the Prior Art
It is known from German Offenlegungsschrift No. 2,421,900 to use as an impact strength improving component with PVC, graft polymer granules prepared by grafting vinyl chloride onto granulated ethylene vinyl acetate copolymers in the gel phase and having a high content of ethylene, e.g., between 85 to 97 weight-percent. Dilauroyl peroxide is used as the catalyst in Example A. The homogeneous incorporation of such granulated gel graft polymers into PVC, however, involves considerable difficulty, especially if the operation is to be conducted on a technical scale. For example, depending on the amount of graft copolymer used and on its chlorine content, quite specific process conditions must be maintained if reproducible and optimum results are to be achieved, especially with regard to the surface quality of the resulting products.
If it is desired to achieve products having smooth surfaces, it is necessary, when graft copolymer granules are used as components of the composition, to subject the PVC compositions to a granulating process and to make these as an impact strength improving component with PVC granules into the formed products.
In experiments it has been found that products of good surface quality can be arrived at by using powdered ethylene vinyl acetate or ethylene alkyl acrylate graft copolymers as compounding components for PVC or vinyl chloride copolymers. Nevertheless, powdered ethylene vinyl acetate and ethylene alkyl acrylate copolymers cannot be grafted with vinyl chloride in the same way as the granulated products.
If, for example, granules of ethylene-vinyl acetate copolymers are used as the basic polymer (e.g., granules of a length of about 3.8 to 4.2 mm and a diameter of about 2.8 to 2.9 mm) and lauroyl peroxide, for example, is used as the initiator, the transfer of the grafting operation from the laboratory scale using a one-liter autoclave, for example, to a layer scale, e.g., a 30-liter or 100-liter autoclave, involves no difficulties. By the continued injection of vinyl chloride, without the further addition of lauroyl peroxide, graft copolymers can be prepared on a semitechnical or technical scale having contents of up to 60 to 70% of vinyl chloride incorporated by polymerization.
If, however, ethylene-vinyl acetate copolymers are used in powder form, e.g., with grain sizes smaller than 300 microns, and the same enlargement of the scale is attempted, considerable difficulty is encountered. Although the production of graft polymers using powdered ethylene copolymers is successful in the one-liter autoclave, if the same process conditions (circulation temperature of, e.g., 68.degree. C. and dilauroyl peroxide initiator) are applied to a larger batch, such as a 30-fold batch using a 30-liter autoclave, the polymerization will come to a stop in the 30-liter autoclave at a low degree of transformation. It is no longer possible to continue the grafting of vinyl chloride onto the powdered basic polymer without the addition of more initiator. If additional vinyl chloride containing the initiator in solution is pumped in, further polymerization will take place. Nevertheless, the fabricating qualities of the compositions prepared from these graft copolymers and PVC are not satisfactory in practice. As a result of the poor working qualities of the molding compositions, moldings are produced which contain numerous inhomogeneities.
If the same experiment is performed in an autoclave having a thirty-liter capacity, but using dicetyl percarbonate as initiator at a circulation temperature of 45.degree. C., the transformations become higher, but again products are obtained whose working characteristics are not adequate for commercial purposes.
If this experiment is performed, not in a thirty-liter autoclave, but in a 100-liter autoclave with correspondingly larger amounts, the transformation becomes further reduced. If it is performed on a still larger scale, such as for example in a two cubic meter autoclave with a corresponding quantity, the above-described difficulties become considerably greater.
It is an object of this invention, therefore, to provide a commercially suitable process for the preparation of graft copolymers. It is a further object of this invention to provide a commercially suitable process which can be carried out in large vessels to prepare valuable polyvinyl chloride graft polymers of basic polymers which are in powder form.
It is a further object of this invention to provide improved polyvinyl chloride graft polymers which, when combined with polyvinyl chloride homo-or copolymers, provide improved impact strength, U-notch test values and rigidity.
These and other objects of this invention will become apparent from the following description and claims.